In high-speed optical communication such as 100 Gigabit Ethernet (registered trademark), standardized optical modules such as CFPx and QSFP 28 are adopted by client side terminals. In order to achieve desired transmission speeds, these optical modules adopt a transmission scheme using amplitude modulation (AM) with 25 Gbps×4 channels (4 wavelengths). In the receiver optical subassembly (ROSA) used for photoelectric conversion and amplification at the receiver's side, four channels are integrated in one package for downscaling.
In the ER4 standard for up to 40 km transmission, among the standard optical modules, a semiconductor optical amplifier (SOA) and a variable optical attenuator (VOA) are used in order to secure a dynamic range for the optical reception power from optical power loss. At the time of receiving low power signals, the received low power signals are collectively amplified by the SOA; at the time of receiving high power signals, the received high power signals are collectively attenuated by the VOA, are demultiplexed to each channel, and are then received by a PIN photodiode (PD).
However, for an objective of downscaling CFP 4 and QSFP 28, the SOA and VOA may be difficult to be disposed due to constraints of mounting sizes. Accordingly, a light receiving scheme using an avalanche photodiode (APD) is being adopted. APD amplifies photocurrent by application of reverse bias (voltage) to achieve high photosensitivity.